The present invention relates to an apparatus for the separate discharge of letters and similar flat items from a stack of such items, the apparatus being of the type including a movable support supporting the stack of items, a rotating withdrawal mechanism in engagement with the foremost item in the stack, means defining a supporting wall manually movable into an initial position for supporting the trailing end of the stack, a motor connected to drive the supporting wall toward the withdrawal mechanism under control of a switch actuated in dependence upon the force being exerted by the stack on the withdrawal mechanism.
The movable support in such apparatus is usually a movably mounted endless base belt which is likewise driven in dependence on the actuation on the abovementioned switch in the direction toward the separating belt, generally at the same speed as the supporting wall. Such a device is disclosed, for example, in U.S. Pat. No. 3,981,493 and is shown in FIGS. 1 and 5 thereof.
Controlling the drive for the supporting wall in dependence on the force exerted by the stack on the withdrawal mechanism has the purpose of keeping this force essentially constant. This is necessary because the withdrawal mechanism operates reliably, i.e., with a minimum of removal errors, in particular a minimum of double removals, only within a certain range of contact force.
For structural reasons, however, in the known devices the above object is not attained under certain operating conditions. The speed at which the supporting wall and the base belt are to be advanced, or driven, should be selected high enough that even if successive thick items of mail are discharged the stack will follow rapidly enough. Since on the other hand, the effective on period of the drive cannot be made less than about 0.1 second, the minimum attainable incremental displacement of the supporting wall and the base belt during each switch-on phase is, for example, about 2 to 3 mm. This may lead to difficulties during discharge of successive thin items, such as postcards, for example. Due to the slip between the stack and the base belt, in conjunction with the on-off hysteresis of the microswitch based on the stack contact force, a plurality of such minimum displacements may be combined with the result that the supporting wall undergoes a displacement greater than that corresponding to the reduction in stack length due to the removal of the thin items. The resulting temporary compaction, associated with an increase of the stack contact force, with decreasing stack length, or if the stack contains essentially only thin and hard items, may have the result that the pressure force exerted by the stack against the withdrawal mechanism takes on unduly high values and produces operating malfunctions.